1. Field of the Invention
The present invention relates to optical head apparatus for optical disks that is suited for the recording and reading of short wavelength laser light.
2. Description of the Prior Art
With the advent of the electronic information age, the quantity of information that has to be handled has recently been growing at a tremendous pace. This has led to an increase in research and development work related to optical disks, which offer high storage density, large capacity and random access capability. There are read-only optical disks, write-once optical disks, and erasable optical disks. The read-only type has already been made into a commercial product in the form of the Company Disk.
In write-once disk systems and erasable disk systems, a beam of laser light is used to record information on, and read information from, a disk that is being rotated by a spindle. In most cases, the laser light source is a laser diode, a device that is lightweight and compact enough to be fabricated on a chip that measures 0.5 mm or less.
FIG. 3 is a drawing showing the general construction of the type of conventional optical head apparatus that is used for recording and reading information on write-once optical disks and erasable optical disks. Typically, the light source is an AlGaAs laser diode 1 that outputs infrared light having a wavelength of 730 nm to 830 nm which, together with various optical components such as servos for focusing and tracking, forms a slider member 18 that moves axially with respect to the spinning disk 20 for recording and reading information at specific locations on the disk 20.
When information is to be recorded, the laser diode 1 is operated by a drive circuit (not shown) to emit a beam of laser light in accordance with the information to be recorded. The laser beam thus emitted is divergent, and it is therefore first passed through a collimator lens 12 to form it into a parallel beam which, after passing through a beam splitter 18a and a right-angle prism 18b, impinges on a focusing lens 18c which condenses the incident laser light. If the aperture number of the focusing lens 18c is NA, for a laser beam of wavelength .lambda., the diameter d of the laser beam coming from the focusing lens 18c that will be projected at specific locations on the spinning disk 20 will be: EQU d=0.82.lambda./NA (1)
The laser beam impinging on the disk 20 melts and deforms the recording layer, or changes it from an amorphous state to a crystalline state or, in thee case of a magneto-optical disk, reverses the orientation of the magnetization, thereby recording the desired information on the disk 20 as a pattern of information-carrying regions that are referred to as pits.
Information thus recorded is read form the disk by using a laser beam of a lower power than the laser beam used for the recording. The laser beam is reflected from the disk and passes through the beam splitter 18a and condenser lens 14 and impinges on a photodetector 18d, which detects the changes in reflectance between pits and non-pit portions which corresponds to the information to be reproduced.
However, with the recent remarkable progress in the technology of semiconductor memories, magnetic memories and other such memory media, there has arisen an urgent need to also increase the recording density of optical disks.
The recording density of write-once disks and erasable disks is defined by the size of the pits, while the size of the pits is, in turn, determined by the diameter of the laser beam used in the recording process. The minimum beam diameter is set by the diffraction limit of the light, and as shown by equation (1), this is proportional to the wavelength .lambda. of the laser light impinging on the focusing lens. It follows, therefore, that the wavelength of the laser light has to be shortened in order to raise the recording density of optical disks.
At present, however, the shortest wavelength that can be achieved with the small, lightweight laser diodes in use is 670 nm, using an AlGaInP laser diode. To obtain laser light having a wavelength shorter than 600 nm would require the use of a large argon gas laser, which would make the system much larger.
One solution that has been tried is to have a laser diode as the light source and use a non-linear optical crystal to reduce the wavelength of the light generated by the laser diode. However, the optical components have to be mounted on the slider member. This has an adverse affect on system performance, because it increases the weight and size of the slider member, slowing its access speed.